Brightness Temperature Contrast Research Articles

Validation of the Cloud_CCI (Cloud Climate Change Initiative) cloud products in the Arctic

Abstract. The role of clouds in the Arctic radiation budget is not well understood. Ground-based and airborne measurements provide valuable data to test and improve our understanding. However, the ground-based measurements are intrinsically sparse, and the airborne observations are snapshots in time and space. Passive remote sensing measurements from satellite sensors offer high spatial coverage and an evolving time series, having lengths potentially of decades. However, detecting clouds by passive satellite remote sensing sensors is challenging over the Arctic because of the brightness of snow and ice in the ultraviolet and visible spectral regions and because of the small brightness temperature contrast to the surface. Consequently, the quality of the resulting cloud data products needs to be assessed quantitatively. In this study, we validate the cloud data products retrieved from the Advanced Very High Resolution Radiometer (AVHRR) post meridiem (PM) data from the polar-orbiting NOAA-19 satellite and compare them with those derived from the ground-based instruments during the sunlit months. The AVHRR cloud data products by the European Space Agency (ESA) Cloud Climate Change Initiative (Cloud_CCI) project uses the observations in the visible and IR bands to determine cloud properties. The ground-based measurements from four high-latitude sites have been selected for this investigation: Hyytiälä (61.84∘ N, 24.29∘ E), North Slope of Alaska (NSA; 71.32∘ N, 156.61∘ W), Ny-Ålesund (Ny-Å; 78.92∘ N, 11.93∘ E), and Summit (72.59∘ N, 38.42∘ W). The liquid water path (LWP) ground-based data are retrieved from microwave radiometers, while the cloud top height (CTH) has been determined from the integrated lidar–radar measurements. The quality of the satellite products, cloud mask and cloud optical depth (COD), has been assessed using data from NSA, whereas LWP and CTH have been investigated over Hyytiälä, NSA, Ny-Å, and Summit. The Cloud_CCI COD results for liquid water clouds are in better agreement with the NSA radiometer data than those for ice clouds. For liquid water clouds, the Cloud_CCI COD is underestimated roughly by 3 optical depth (OD) units. When ice clouds are included, the underestimation increases to about 5 OD units. The Cloud_CCI LWP is overestimated over Hyytiälä by ≈7 g m−2, over NSA by ≈16 g m−2, and over Ny-Å by ≈24 g m−2. Over Summit, CCI LWP is overestimated for values ≤20 g m−2 and underestimated for values >20 g m−2. Overall the results of the CCI LWP retrievals are within the ground-based instrument uncertainties. To understand the effects of multi-layer clouds on the CTH retrievals, the statistics are compared between the single-layer clouds and all types (single-layer + multi-layer). For CTH retrievals, the Cloud_CCI product overestimates the CTH for single-layer clouds. When the multi-layer clouds are included (i.e., all types), the observed CTH overestimation becomes an underestimation of about 360–420 m. The CTH results over Summit station showed the highest biases compared to the other three sites. To understand the scale-dependent differences between the satellite and ground-based data, the Bland–Altman method is applied. This method does not identify any scale-dependent differences for all the selected cloud parameters except for the retrievals over the Summit station. In summary, the Cloud_CCI cloud data products investigated agree reasonably well with those retrieved from ground-based measurements made at the four high-latitude sites.

Floating Oil On Seawater, in Situ Thermal Structure, Thickness, and Remote Sensing: Part 1. Laboratory Study

Abstract Thermal infrared (TIR) remote sensing shows potential for oil spill response. Most TIR remote sensing uses the brightness temperature contrast, ΔTB, between oil and oil-free water. This study evaluates the potential of remotely-sensed ΔTB to quantify oil thickness through a series of lab experiments that measured actual surface temperature contrast, ΔT, for a Denver/Julesburg Basin crude oil. Specifically, TIR and visible video cameras imaged oil layers of different thicknesses, h, floating on seawater. Also, fast thermocouples collected high resolution (to ~10 µm) vertical profiles. A novel deconvolution approach corrected for thermocouple time response. Slope changes in the profiles indicated the oil-water interface location. Experimental illumination was for full sunlight (outdoors) and incandescent light (indoors) for emulsified and unemulsified crude oils. Oil slicks were classified by distinct behaviors with a transition at h~1 mm. Thinner than this transition, ΔT was strongly sensitive to h, with oil temperatures decreasing monotonically with depth in the slick. In contrast, oil slicks thicker than this transition featured an internal temperature peak with ΔT weakly sensitive to h. This peak isolates the oil below the peak from affecting surface temperatures. Thicker slicks also were associated with a thin warm air layer that increasingly developed with h. This study highlights the potential for deriving oil slick thickness from ΔTB and the need for an improved understanding of solar insolation absorption and heat transfer for a range of oil and oil emulsion slicks.

Measuring Floating Thick Seep Oil from the Coal Oil Point Marine Hydrocarbon Seep Field by Quantitative Thermal Oil Slick Remote Sensing

Remote sensing techniques offer significant potential for generating accurate thick oil slick maps critical for marine oil spill response. However, field validation and methodology assessment challenges remain. Here, we report on an approach to leveraging oil emissions from the Coal Oil Point (COP) natural marine hydrocarbon seepage offshore of southern California, where prolific oil seepage produces thick oil slicks stretching many kilometers. Specifically, we demonstrate and validate a remote sensing approach as part of the Seep Assessment Study (SAS). Thick oil is sufficient for effective mitigation strategies and is set at 0.15 mm. The brightness temperature of thick oil, TBO, is warmer than oil-free seawater, TBW, allowing segregation of oil from seawater. High spatial-resolution airborne thermal and visible slick imagery were acquired as part of the SAS; including along-slick “streamer” surveys and cross-slick calibration surveys. Several cross-slick survey-imaged short oil slick segments that were collected by a customized harbor oil skimmer; termed “collects”. The brightness temperature contrast, ΔTB (TBO − TBW), for oil pixels (based on a semi-supervised classification of oil pixels) and oil thickness, h, from collected oil for each collect provided the empirical calibration of ΔTB(h). The TB probability distributions provided TBO and TBW, whereas a spatial model of TBW provided ΔTB for the streamer analysis. Complicating TBW was the fact that streamers were located at current shears where two water masses intersect, leading to a TB discontinuity at the slick. This current shear arose from a persistent eddy down current of the COP that provides critical steering of oil slicks from the Coal Oil Point. The total floating thick oil in a streamer observed on 23 May and a streamer observed on 25 May 2016 was estimated at 311 (2.3 bbl) and 2671 kg (20 bbl) with mean linear floating oil 0.14 and 2.4 kg m−1 with uncertainties by Monte Carlo simulations of 25% and 7%, respectively. Based on typical currents, the average of these two streamers corresponds to 265 g s−1 (~200 bbl day−1) in a range of 60–340 bbl day−1, with significant short-term temporal variability that suggests slug flow for the seep oil emissions. Given that there are typically four or five streamers, these data are consistent with field emissions that are higher than the literature estimates.

An inverse planned oil release validation method for estimating oil slick thickness from thermal contrast remote sensing by in-scene calibration

This study demonstrates a method to estimate floating oil slick thickness based on remote sensing of thermal infrared contrast. The approach was demonstrated for thick oil slicks from natural seeps in the Coal Oil Point seep field, offshore southern California. Airborne thermal infrared and visible spectrum remote sensing imagery were acquired along with position and orientation data by the SeaSpires™ science package. Remote sensing data were acquired in the cross-slick direction of oil slick segments that were targeted for collection, termed “collects.” A collect consisted of booming, skimming, and offloading the oil slick segment into buckets for analysis at the laboratory. Each collect provided an in-scene calibration point of oil thickness versus brightness temperature contrast, ΔTB, where TB is the sensor-reported temperature based on the emitted thermal radiation and differs from the true temperature due to the oil's emissivity. ΔTB is the TB difference between the oil and oil-free sea surface. Thus, this study is a reverse planned oil-release experiment that demonstrates the value of natural seeps for oil spill science.• Novel approach to quantify floating oil thickness• Custom modified weir skimmer used with added floor and structural strengthening

Phase Curves of Hot Neptune LTT 9779b Suggest a High-metallicity Atmosphere

Abstract Phase-curve measurements provide a global view of the composition, thermal structure, and dynamics of exoplanet atmospheres. Although most of the dozens of phase-curve measurements made to date are of large, massive hot Jupiters, there is considerable interest in probing the atmospheres of the smaller planets that are the more typical endproduct of the planet formation process. One such planet that is favorable for these studies is the ultrahot Neptune LTT 9779b, a rare denizen of the Neptune desert. A companion paper presents the planet’s secondary eclipses and dayside thermal emission spectrum; in this work we describe the planet’s optical and infrared phase curves, characterized using a combination of Spitzer and Transiting Exoplanet Survey Satellite (TESS) photometry. We detect LTT 9779b’s thermal phase variations at 4.5 μm, finding a phase amplitude of 358 ± 106 ppm and no significant phase offset, with a longitude of peak emission occurring −10° ± 21° east of the substellar point. Combined with our secondary eclipse observations, these phase-curve measurements imply a 4.5 μm dayside brightness temperature of 1800 ± 120 K, a nightside brightness temperature of 700 ± 430 K (<1350 K at 2σ confidence), and a day–night brightness temperature contrast of 1110 ± 460 K. We compare our data to the predictions of 3D general circulation models calculated at multiple metallicity levels and to similar observations of hot Jupiters experiencing similar levels of stellar irradiation. Though not conclusive, our measurement of its small 4.5 μm phase offset, the relatively large amplitude of the phase variation, and the qualitative differences between our target’s dayside emission spectrum and those of hot Jupiters of similar temperatures all suggest a supersolar atmospheric metallicity for LTT 9779b, as might be expected given its size and mass. Finally, we measure the planet’s transits at both 3.6 μm and 4.5 μm, providing a refined ephemeris (P = 0.79207022 ± 0.00000069 days, T 0 = 2458783.51636 ± 0.00027, BJDTDB) that will enable efficient scheduling of future observations to further characterize the atmosphere of this intriguing planet.

CONSTRAINING POLARIZED FOREGROUNDS FOR EoR EXPERIMENTS. I. 2D POWER SPECTRA FROM THE PAPER-32 IMAGING ARRAY

ABSTRACT Current generation low-frequency interferometers constructed with the objective of detecting the high-redshift 21 cm background aim to generate power spectra of the brightness temperature contrast of neutral hydrogen in primordial intergalactic medium. Two-dimensional (2D) power spectra (power in Fourier modes parallel and perpendicular to the line of sight) that formed from interferometric visibilities have been shown to delineate a boundary between spectrally smooth foregrounds (known as the wedge) and spectrally structured 21 cm background emission (the EoR window). However, polarized foregrounds are known to possess spectral structure due to Faraday rotation, which can leak into the EoR window. In this work we create and analyze 2D power spectra from the PAPER-32 imaging array in Stokes I, Q, U, and V. These allow us to observe and diagnose systematic effects in our calibration at high signal-to-noise within the Fourier space most relevant to EoR experiments. We observe well-defined windows in the Stokes visibilities, with Stokes Q, U, and V power spectra sharing a similar wedge shape to that seen in Stokes I. With modest polarization calibration, we see no evidence that polarization calibration errors move power outside the wedge in any Stokes visibility to the noise levels attained. Deeper integrations will be required to confirm that this behavior persists to the depth required for EoR detection.

THE 4.5 μm FULL-ORBIT PHASE CURVE OF THE HOT JUPITER HD 209458b

The hot Jupiter HD 209458b is particularly amenable to detailed study as it is among the brightest transiting exoplanet systems currently known (V-mag = 7.65; K-mag = 6.308) and has a large planet-to-star contrast ratio. HD 209458b is predicted to be in synchronous rotation about its host star with a hot spot that is shifted eastward of the substellar point by superrotating equatorial winds. Here we present the first full-orbit observations of HD 209458b, in which its 4.5 $\mu$m emission was recorded with $Spitzer$/IRAC. Our study revises the previous 4.5 $\mu$m measurement of HD 209458b's secondary eclipse emission downward by $\sim$35% to $0.1391%^{+0.0072%}_{-0.0069%}$, changing our interpretation of the properties of its dayside atmosphere. We find that the hot spot on the planet's dayside is shifted eastward of the substellar point by $40.9^{\circ}\pm{6.0^{\circ}}$, in agreement with circulation models predicting equatorial superrotation. HD 209458b's dayside (T$_{bright}$ = 1499 $\pm$ 15 K) and nightside (T$_{bright}$ = 972 $\pm$ 44 K) emission indicates a day-to-night brightness temperature contrast smaller than that observed for more highly irradiated exoplanets, suggesting that the day-to-night temperature contrast may be partially a function of the incident stellar radiation. The observed phase curve shape deviates modestly from global circulation model predictions potentially due to disequilibrium chemistry or deficiencies in the current hot CH$_{4}$ line lists used in these models. Observations of the phase curve at additional wavelengths are needed in order to determine the possible presence and spatial extent of a dayside temperature inversion, as well as to improve our overall understanding of this planet's atmospheric circulation.

Impact of cirrus clouds heterogeneities on top-of-atmosphere thermal infrared radiation

Abstract. This paper presents a study of the impact of cirrus cloud heterogeneities on the thermal infrared brightness temperatures at the top of the atmosphere (TOA). Realistic 3-D cirri are generated by a cloud generator based on simplified thermodynamic and dynamic equations and on the control of invariant scale properties. The 3-D thermal infrared radiative transfer is simulated with a Monte Carlo model for three typical spectral bands in the infrared atmospheric window. Comparisons of TOA brightness temperatures resulting from 1-D and 3-D radiative transfer show significant differences for optically thick cirrus (τ > 0.3 at 532 nm) and are mainly due to the plane-parallel approximation (PPA). At the spatial resolution of 1 km × 1 km, two principal parameters control the heterogeneity effects on brightness temperatures: i) the optical thickness standard deviation inside the observation pixel, ii) the brightness temperature contrast between the top of the cirrus~and the clear-sky atmosphere. Furthermore, we show that the difference between 1-D and 3-D brightness temperatures increases with the zenith view angle from two to ten times between 0° and 60° due to the tilted independent pixel approximation (TIPA).

Effects of noise on optimal deconvolution accuracy in microwave observations

Due to large footprints of remotely sensed microwave brightness temperatures, accuracy of microwave observations in areas of large surface heterogeneity has always been a technological challenge. Microwave observations in areas dominated by waterbodies typically exhibit observed brightness temperature several tens of kelvins lower than areas having no surface water. The non-linearity between brightness temperature and other geophysical quantities such as soil moisture makes the accuracy of microwave observations a critical element for accurate estimation of these quantities. In retrieving soil moisture estimates, an error of 1 K in remotely sensed microwave brightness temperatures results in about 0.5–1% error in volumetric soil moisture. Large uncertainties in the observed brightness temperatures make such observations unusable in areas of large brightness temperature contrast. In this article, we discuss a deconvolution method to improve accuracy using the overlap in the adjacent microwave observations. We have shown that the method results in improved accuracy of 40% in brightness temperature estimation in regions of high brightness temperature contrast.

MMW Radiometric Characteristic Experiments of Water Plant for Environmental Protection

Different from the usual plant radiometric measurement, the antenna temperature contrast modeling of plant and the water surface is presented for the purpose of plant growth monitoring. With the help of multilayer dielectric model, the brightness temperature contrast of the plant on water is obtained by modeling and experiment. According to the actual measurement by 3mm and 8mm band radiometer, the contrast of water and plant on the water can be test effectively for the environmental protection of the water surface.

Ionized bubble number count as a probe of non-Gaussianity

The number count of ionized bubbles on a map of 21 cm fluctuations with the primordial non-Gaussianity is investigated. The existence of the primordial non-Gaussianity modifies the reionization process, because the formation of collapsed objects, which could be the source of reionization photons, is affected by the primordial non-Gaussianity. In this paper, the abundance of ionized bubbles is calculated by using a simple analytic model with the local type of the primordial non-Gaussianity, which is parameterized by f_{NL}. In order to take into account the dependence of the number count on the size of ionized bubbles and the resolution of the observation instrument, a threshold parameter B_b which is related to the the surface brightness temperature contrast of an ionized bubble is introduced. We show the potential to put the constraint on f_{NL} from the number count by future observations such as LOFAR and SKA.

Detection of Cirrus Clouds Using Infrared Radiometry

The detection of low-level clouds from ground-based infrared (IR) radiometry is usually based on the IR brightness temperature (IRBT) contrast between the warm clouds and the cold clear-sky background. This method works as long as the brightness temperature contrast subsists. It is not the case for cirrus clouds, which are usually optically thin and exhibit a low brightness temperature, comparable to that of clear sky in similar atmospheric conditions. In this paper, we propose a detection scheme to discriminate between clear sky and cirrus sky based on the fluctuations of the IRBT rather than on the absolute IRBT values. For this, we use the detrended fluctuation analysis (DFA) method on IRBT data when no liquid clouds were present. We compute the exponent α for nine IRBT time series covering clear- and cirrus-sky situations. We find that, for the fluctuation range of 1-5 min, α ≃ 0.1 for clear sky, α ≃ 0.5 for stratiform cirrus layers, and α ≃ 1 for broken cirrus layers. We suggest a threshold value of α ≃ 0.25 for the discrimination between clear sky and cirrus sky. We also examine the relationship between DFA and the classical spectral analysis and find that spectral analysis could be an alternative to the DFA method in detecting the presence of cirrus clouds.

Deriving Winds at Cloud-Base Height With an Infrared Camera

An uncooled commercial infrared camera is used to retrieve horizontal winds at cloud-base height. The camera is equipped with a microbolometer array of 320 times 240 pixels covering a field of view of 32deg times 24deg. It operates in the atmospheric window from 7.5 to 14 mum . In this wavelength range, the camera has day and night measurement capabilities, independent of the illumination conditions. Optically thick clouds appear as blackbody radiators and offer a large brightness temperature contrast with the much colder clear-sky background. We develop a method to follow the infrared structures in the clouds as they pass through the field of view of the camera. Taking a picture every 6 s, the angular velocity of the cloud base is calculated as the ratio of the angular displacement of the cloud structures between two pictures over the elapsed time. To transform the angular velocity into a metric velocity, we use the temperature profile retrieved by a microwave radiometer called All-Sky MUlti-WAvelength Radiometer. The case study of September 10, 2007 is shown. The retrieved wind is of 18.0 plusmn 2.4 m/s at an altitude of 4164 plusmn 546 m above the instrument, blowing from northwest. These results agree well with radiosondings launched in the vicinity of the experiment site.

広島市市街地におけるランドサットデータによる輝度温度と気温および都市キャノピー層の構造との対応関係

Thermal infrared imageries of the LANDSAT Thematic Mapper (TM) band 6 are compared to the air temperature distributions and the geometry and materials of the urban area in Hiroshima City (132.5 °E, 34.4 °N), Japan, to extract the information on the urban heat island from the images. The results are summarized as follows :(1) Surface heat island is recognized on the LANDSAT TM nighttime thermal image taken at 21 JST on May 17, 1991. The distribution of brightness temperature matches closely with the air temperature distribution at 21 JST on MAY 19, 1992 (thermal image at that time was not able to be taken because of clouds). The discrepancy between air temperature and brightness temperature, recognized in the southern part of the city, can be explained well by the advection caused by the land breeze.(2) The daytime thermal images, which was taken at 10 JST on May 21, 1992, shows that brightness temperature at the central part of the urban area is low compared with the suburban area. The part of low temperature is also recognized in the air temperature distribution observed at the same time of the satellite overpass.(3) The distribution maps of height, surface area, volume and lot size of buildings are produced to compare the brightness temperature with the structure of the urban area. Brightness temperature is high along the major roads in the nocturnal thermal image. These parts correspond to the area where the buildings with high roof level, large lot size, large volume and large surface area are distributed. In these areas, daytime storage of heat and release of it at evening is considered to be important.(4) The interior of the blocks surrounding to the major roads shows relatively low brightness temperature. The area corresponds to the densely built-up area with low roof buildings or the open spaces such as school grounds and parks. Rapid radiative cooling in these areas is important to the formation of brightness temperature contrast in the blocks.

The detectability of sea-ice leads in satellite data as a function of atmospheric conditions and measurement scale

The release of heat from sea-ice leads is an important component of the heat budget in the Arctic, but the impact of leads on regional scale climate is difficult to assess without information on their distribution in both space and time. Remote sensing of leads using satellite data, specifically AVHRR thermal and Landsat visible imagery, is examined with respect to one lead parameter: lead width. Atmospheric effects are illustrated through the concept of thermal contrast transmittance, where the brightness temperature contrast between leads of various ice thicknesses and the surrounding multi-year ice is simulated using a radiative transfer model. Calculations are made as a function of aerosol, ice crystal precipitation, and cirrus cloud optical depths. For example, at ice crystal optical depths of more than about 1.5 under mean January conditions in the central Arctic, the brightness temperature differences between 2 m and 5 cm thick ice are similar to the ice temperature variability so that there would be insufficient contrast to distinguish a lead from the surrounding ice. The geometrical aspects of the sensor are also simulated by degrading Landsat data so that the effect of sensor field-of-view on retrieved lead width statistics can be assessed. Large leads tend to “grow” with increased pixel size while small leads disappear. Changes in lead width and orientation distributions can readily be seen.

The detectability of sea-ice leads in satellite data as a function of atmospheric conditions and measurement scale

The release of heat from sea-ice leads is an important component of the heat budget in the Arctic, but the impact of leads on regional scale climate is difficult to assess without information on their distribution in both space and time. Remote sensing of leads using satellite data, specifically AVHRR thermal and Landsat visible imagery, is examined with respect to one lead parameter: lead width. Atmospheric effects are illustrated through the concept of thermal contrast transmittance, where the brightness temperature contrast between leads of various ice thicknesses and the surrounding multi-year ice is simulated using a radiative transfer model. Calculations are made as a function of aerosol, ice crystal precipitation, and cirrus cloud optical depths. For example, at ice crystal optical depths of more than about 1.5 under mean January conditions in the central Arctic, the brightness temperature differences between 2 m and 5 cm thick ice are similar to the ice temperature variability so that there would be insufficient contrast to distinguish a lead from the surrounding ice. The geometrical aspects of the sensor are also simulated by degrading Landsat data so that the effect of sensor field-of-view on retrieved lead width statistics can be assessed. Large leads tend to “grow” with increased pixel size while small leads disappear. Changes in lead width and orientation distributions can readily be seen.